Understanding Fatigue Failures

Datasheets for most materials list a fatigue strength. Depending on the material, this represents either the level of stress below which fatigue does not occur (sometimes called the endurance limit), or the level of stress at which fatigue failure occurs in a given, usually large, number of cycles.

Fatigue strengths listed on datasheets are usually determined by means of fully reversed loading. This means that, during the test, the stress varies from a certain value in tension to an equal and opposite value in compression. This is the how the stress varies in a rotating shaft under a bending load, for example. However, it may not be the way the stress varies in your application. For instance, in your application, the load may repeatedly go from zero to a given value. Or your part may have a certain static tensile load, plus a vibration, which causes the load to oscillate around this value.

In these cases, you should not simply compare the maximum stress in the part to the fatigue strength. Remember, fatigue is all about cyclic stresses -- stresses that vary over time. If the stress doesn't fluctuate, fatigue can't happen. So in addition to the maximum stress, you also need to know the minimum stress at the same location; that is, the range of stresses in a given location over time. Another way of approaching this is to consider an average or steady stress, and an alternating stress, which is the amount by which the stress varies around this value over time.

There are a number of different equations for converting maximum and minimum stress values (or average and alternating stress values) to an equivalent fully reversed stress. The fact that there are so many different equations suggests that none of the equations works perfectly all the time; after all, if one of the equations worked, everyone would simply use that one. You can find these equations in any mechanical engineering textbook. One of the simplest and best known is the Goodman equation. However, I've found that, in most cases, the Walker equation is the most accurate.

As you can see, designing for fatigue is not as simple as designing for overload. This probably helps to explain why fatigue failures are so much more common than overload failures. However, observing these basic principles can help to avoid fatigue failures.

@Beth: We try to do the best we can using published data, while at the same time accumulating our own body of test data about the materials we use most often. This way we don't hold up the development cycle waiting for test results; we just constantly fill in the gaps in our knowledge. I suspect that this is what a lot of other companies do, too.

We also try to avoid component-level fatigue testing where possible. For example, if you understand how die-cast 380 aluminum behaves in fatigue (and the effect of casting porosity on the properties), you can apply that knowledge to any die-cast 380 component; you don't have to repeat the test for each part. But for some components, like crankshafts, component-level testing is a must.

That extra info will be extremely helpful to the community. Thanks so much for providing it, Dave. Your point about having to do your own testing rather than rely on third-party material stuck me, however. Given that so many companies across industries are dealing with time-to-market pressures and there are so many aspects of a product that need vertification and testing, how do you prioritize what's required for fatigue testing, vs. say what's necessary for structural testing or something else? Can development teams really afford to take the time to do this, but I guess the bigger question is can they afford to not?!!!

@Beth: There are a number of CAE tools for fatigue analysis. I don't have direct experience with any of them, but some I've heard of include fe-safe, MSC Fatigue, and nCode. I'm sure there are others. Some companies have their own in-house fatigue codes.

As far as sources for fatigue data, the American Iron and Steel Institute has an excellent, free, on-line database of fatigue data for a wide variety of steels. (Registration is required). Of course, it's important to understand what you're looking at. Not only do you need to find the data for the correct steel; it also needs to be the correct condition. I've seen engineers use data for a given steel in the annealed (soft) condition when the actual part is hardened, and vice versa.

Often, though, it's worthwhile to do your own testing rather than relying on published data. The main drawback of this is that a solid fatigue testing program can take months to complete. For example, at 30 Hz -- which is pretty fast for a hydraulic load frame -- a single 20 million cycle test will take over a week, and you need to perform several such tests in order to have statistically valuable data. The advantage, though, is that you can test the alloys you actually use, in the conditions you actually use them.

Really great explanation on the basic principles that cause fatigue failures--one which even non-engineers like myself can understand. Beyond the key equations you provided at the end of your piece, are there other sources of information about materials properties or specific design tools (perhaps CAE software) that can help engineers with this design challenge?

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